Combusters for heat engines that utilize high frequency wave such as microwaves during ignition, such as those disclosed in Patent References 1, 2 and 3, have been proposed. However, in the combusters disclosed in these publications, the microwave antenna differs in shape and dimensions from an ordinary spark plug, and thus cannot be accommodated in the installation space for an ordinary spark plug. That is, the use of a microwave antenna requires a specialized design for the combuster, and this represents an obstacle to widespread use.
Accordingly, a system in which microwaves are guided into the combustion chamber and ignition is carried out directly with microwaves represents an attempt to address limitations relating to shape and dimensions.
The device disclosed in Patent Reference 4 is furnished internally with a coaxially disposed waveguide structure. One end of an inside conductor of the coaxial waveguide structure projects out towards the combustion chamber, while the other end constitutes the high frequency wave input end.
This input end is provided with a coupling location for an electrical energy supply line. It is possible to coaxially connect an electrical energy supply line and a center conductor to this coupling location through an inductive or capacitive coupling, thereby making it possible for electrical energy to be supplied into the waveguide structure, while achieving electrical integrity.
A system furnished with an auxiliary electrode represents another attempt to address limitations relating to shape and dimensions.
The inventors have succeeding in developing an ignition plug by drawing attention to the fact that, when a plasma is generated from air or the like, OH radicals, ozone, or other active or chemical species with high oxidative power are generated in the plasma. The ignition plug is designed to generate a plasma from a mixture of an oxidant and a fuel in the combustion chamber of a spark-ignited internal-combustion engine, in order to enhance ignition of the mixture and flame propagation subsequent to ignition, thus improving combustion of the mixture.
Furthermore, as disclosed in Patent Reference 5, the inventors were also successful in reducing the size of the ignition device mechanism employed for plasma-enhanced combustion by employing a design in which the ignition plug which is used for spark ignition and the microwave antenna which is used for plasma generation are integrated with an insulator disposed therebetween.    [Patent Reference 1] Japanese Laid-Open Patent Application 2000-274249    [Patent Reference 2] Japanese Laid-Open Patent Application 2002-295264    [Patent Reference 3] Japanese Laid-Open Patent Application 2002-295259    [Patent Reference 4] Japanese Laid-Open Patent Application 2004-087498    [Patent Reference 5] Japanese Laid-Open Patent Application 2007-113570